Once upon a time a lot of pilots with instructor ratings DID instruct for free (perhaps a little petrol money) because they enjoyed it and had other full-time occupations perhaps in aviation or perhaps not. Obviously an instructor who has no other means of earning income and is hour-building will have to be paid for his/her services and rightly so. The down side is the loss of the flying 'club' instructor and the dominance of the flying 'school' instructor. Too many schools - not enough clubs in my opinion but each to his or her own I guess.